/ Field knowledge · IMC 801.18
“Existing chimney to remain” is not a design.
“Existing chimney to remain” is not a design. When an appliance is disconnected or a new one is connected, the existing chimney or vent has to pass four gates: size, passageway, cleanout, and clearance.
Educational 2024 International Mechanical Code (IMC) 801.18 from Daily Code Talk. Confirm the adopted edition. Distinct from factory-built chimneys (805) and general venting (801). Gas-fired appliances also follow the fuel-gas code and the manufacturer. This is not a flue-size calculator.
- Trigger: an appliance is permanently disconnected, or a new appliance is connected, during a new installation. Then 801.18.1 through 801.18.4 apply.
- Four gates — all of them, not one: size (draft and condensation control); passageway (clean, continuous, undamaged); cleanout (present and accessible); clearance (airspace to combustibles and fireblocking, or a listed liner path).
- Passing only size does not make the system reusable. Prior service does not prove current suitability.
On the set: existing-vent survey, appliances removed / remaining / added, four-gate matrix with drawing location and who verifies in the field. Close unused inlet openings. Do not connect until the concealed passageway has an inspection plan.
Reconnect to existing is the comment
A new water heater or furnace scheduled, and a note that says reconnect to the existing chimney, with no survey of size, condition, cleanout, or clearance. That gap is condensation, leakage, carbon monoxide, and a failed inspection. Fuel conversion and Category IV on an old masonry flue are escalate items.
Source: Daily Code Talk — IMC 801.18 · tool: Existing chimney four gates · size: 801.18.1 · passageway: 801.18.2 · cleanout: 801.18.3 · clearance: 801.18.4 · factory-built: 805 · connectors: 803.
What to send
Photos of the chimney, termination, and cleanout if visible; appliance cut sheets (old and new); fuel and vent category; and whether this is a replacement, an added appliance, or a disconnection.
If the set says “existing chimney to remain”
Send the photos and the new appliance cut sheet. Masterbuild will say which of the four gates still have no evidence — before the reconnection is drawn as a given.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
Masterbuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval. Confirm the adopted edition. The four gates are 2024 IMC 801.18 education from Daily Code Talk — not a flue design. Gas appliances follow the fuel-gas code. This page is not legal advice.